The present invention relates to apparatus for altering the concentration of a pre-selected component of a feedstock. The invention is applicable to the removal of micron and submicron species from a fluid phase but it is to be understood that it is not limited thereto
For the sake of convenience, the invention will be described in relation to cross-flow retention or filtration in which the pre-selected component or specie is removed from the feedstock by transfer through a barrier adapted to pass the component or specie and to retain the remainder of the feedstock. However, it is to be understood that the invention is not limited thereto as it may equally be applied to the reverse situation in which the preselected specie is introduced through the barrier to the feedstock.
Micron and submicron species (for example, molecules, colloids, particles and droplets) in a fluid phase (for example, an aqueous phase) can be removed in a number of different ways depending on the quantity of species present.
For low concentrations, depth filtration is probably the most common method applied. An alternative approach is to use a surface filter, for example the Nucleopore membrane; this membrane effects removal of particulates by a surface sieving mechanism. Other types of surface membranes are available; these rely on an active surface skin which is backed up by a porous support layer. In particulate filtration operations such membranes behave similarly to Nucleopore membranes.
When the concentration of retained species is high, depth and dead-end surface barriers become much less attractive, as the pressure drop necessary to effect filtration increases rapidly with solids accumulation.
It is to overcome this problem that a new area of micron and submicron species retention is being developed. The technique used is cross-flow retention. In cross-flow retention a surface membrane is used and the build-up of a layer of retained species is minimised by applying a fluid shear field to the upstream fluid adjacent to the barrier surface. This can be done either by stirring or by pumping the fluid across the barrier surface.
According to this technique, it is quite feasible to operate at a steady state in which a solution is effectively split into a permeate and a retentate, and at steady state no more species accumulates at the barrier surface, causing it to lose performance. Alternatively, the system can be operated in a batch mode, and in this case the feed solution gradually increases in concentration, and although this leads to a drop in throughput, the drop is far less than would occur in the dead-end mode where all of the species collect on or in the barrier.
Generally, for each membrane application in the fields of ultrafiltration, dialysis and electrodialysis, the permeability of the membrane system is generally limited by the layer of retained species (i.e. the concentration layer or, eventually the gel layer) which is present. According to the present invention, it is preferred that laminar flow be employed to remove the gel or cake from the surface of the membrane.
In the case of laminar flow there is a relationship for a given concentration of a fluid to be treated and for a given membrane. This relationship links flux, shear rate and length of the filter flowpath.
As given by Blatt et al in 1970, in Membrane Science and Technology, the relationship is in respect of laminar flow conditions and for the gel polarized condition (i.e. when increased pressure does not increase flux). The relationship is given as: EQU J.alpha.(.gamma./L).sup.0.33 .alpha.(U.sub.B).sup.0.33 (h.sub.c).sup.-0.33 (L).sup.-0.33
wherein
J is the flux for a given area of membrane PA1 U.sub.B is the velocity of the fluid PA1 h.sub.c is the height or thickness of the filter flowpath PA1 L is the length of the filter flow path PA1 .gamma. is the shear rate PA1 (a) inlet means adapted to admit pressurized feedstock into the apparatus, PA1 (b) outlet means adapted to remove treated feedstock from the apparatus, PA1 (c) one or more feedstock flowpaths between the inlet means and outlet means, PA1 (d) one or more barriers adapted to pass said pre-selected component(s) and having a first surface past which the feedstock is directed, and, PA1 (e) transfer means adapted to communicate with the opposite surface of the or each barrier, PA1 (i) Dialysis is a four vector system for two fluids with apparatus comprising two inlets and two outlets--an inlet and an outlet for the material to be dialysed and a separate inlet and outlet for the dialysing liquid which flows as a counter current to the material to be dialysed. Cross flow filtration, on the other hand, is a three vector system for one fluid--therebeing with only one inlet for the feedstock to be treated and two separate outlets one for the concentrate or retentate and one for the filtrate or permeate PA1 (ii) Dialysis results in a dilution of the material being dialysed whereas cross-flow filtration results in concentration of the retained species in the material being treated. PA1 (iii) Dialysis operates at pressures of less than 10 KPa whereas cross-flow filtration is performed at pressures of the order of 100 KPa. PA1 (iv) Dialysis uses low water flux membranes at low flow rates (e.g. 2 liters/day) with a minimum transmembrane pressure gradient. Cross-flow filtration uses high water flux, highly permeable membranes (flow rates e.g. 50 liters/hour) with a large transmembrane pressure gradient. PA1 (v) Dialysis uses a pair of membranes each of about 40 microns thickness with a channel height or thickness of about 150 microns at normal atmospheric pressure. Comparable cross-flow filtration apparatus uses a pair of membranes each of about 120-200 microns thickness with a zero channel height (i.e. the membranes are in contact) at normal atmospheric pressure, and a channel height or thickness of about 50 microns under an operative transmembrane pressure gradient of 100 KPa.
The shear rate is an expression of the ratio of .nu. the tangential velocity of fluid between adjacent membranes and the height of the filter flowpath or channel, that is: EQU .gamma..alpha.(.nu./hc)
When the gel layer (rather than the porosity of the membrane) is the limiting factor to membrane performance, the flux is linked to the shear rate through this ratio. This means that the effect of reducing the channel height or thickness is to increase significantly both the shear rate and the flux.
Generally, in cross-flow retention, the energy involved for recirculation of the feed is the highest direct cost factor of the operation. For classical systems, with h.sub.c of the order of 1 mm, energy consumption is of the order of 1 kW/square meter of membrane installed. In the case of tubular systems, with tubes of diameter of the order of 1 cm., the cost of energy required is even greater.
Accordingly, there is an interest in developing cross-flow capillary retention or filtration and ultrafiltration apparatus wherein the height of the flowpath is significantly reduced, for example to about 50-100 microns. By using a flowpath height of only this magnitude, the pumping capacity required per square meter of membrane is proportionally reduced; double the channel height and the pumping capacity required is more than doubled.